The invention relates to an internal planar antenna especially applicable in mobile terminals. The invention further relates to a radio device employing an internal planar antenna.
In portable radio devices, mobile terminals in particular, the antenna is preferably placed within the casing of the device for increased comfort of use. There are certain basic electrical requirements for an antenna. Its impedance matching at the operating frequency has to be so good that, as regards matching, the efficiency of radio transmitting and receiving is at an acceptable level. The matching has to apply to the whole frequency band of the radio system, i.e. the antenna bandwidth has to correspond to the band in question. Resistive and dielectric losses in the antenna structure shall naturally be small. Smaller losses mean higher antenna gain and more efficient radiation. The radio device may be designed to function in a plurality of radio systems so that its antenna, too, must have more than one band. It is advantageous for the operation of a portable radio device if it has good antenna transmitting and receiving characteristics in all directions, although this is not necessary. On the other hand, it is considered undesirable that radiation is directed to the user""s head, which imposes an extra requirement for the antenna of a radio device held on the user""s ear.
An antenna with satisfactory characteristics which fits inside a small device is in practice most easily implemented as a planar structure: The antenna comprises a radiating plane and a ground plane parallel thereto. FIG. 1 shows an example of such a known planar antenna. It comprises a circuit board 101 with a conductive layer on the upper surface thereof, which conductive layer serves as a ground plane GND of the antenna. Elevated from the ground plane is a radiating plane 110 in connection with a feed conductor 121 and a short circuit conductor 122 which connects the radiating plane to the ground plane. The antenna is thus a planar inverted F antenna (PIFA). FIG. 1 also shows a portion of a dielectric frame 170 supporting the radiating plane. The radiating plane includes a slot 115 starting from the edge thereof and dividing the radiating plane into two branches of different lengths, as viewed from the short circuit point. Thus the PIFA has got two separate fundamental resonance frequencies and respective operating bands. In the example of FIG. 1 the feed conductor 121 and short circuit conductor 122 are of the spring contact type and constitute a single unitary piece with the radiating plane 110. Each conductor has a part parallel to the radiating plane, which functions as a spring, and a part extending therefrom towards the ground plane. At the lower end there is further a part parallel to the ground plane, comprising the contact proper. When the radiating plane is installed, a spring force presses the contacts against the upper surface of the circuit board 101, the contact of the short circuit conductor against the ground plane, and the contact of the feed conductor against a contact surface 105. This, in turn, is connected to an antenna port.
FIG. 2 shows another example of a known planar antenna. If differs from the example of FIG. 1 only as regards the feed and short circuit arrangements. The short circuit conductor is in this case a straight cylindrical conductor connected to the radiating plane 210 and ground plane GND by means of soldering, for example. It may also form a single piece with the radiating plane. The feed conductor 221, too, is a straight cylindrical conductor connected to the antenna port through a via 206 in the circuit board 201.
The antenna structures described above can be improved in terms of antenna gain e.g. by replacing copper in the planar surfaces with some other surface material having even better conductivity. A disadvantage, then, is that the specific absorption rate (SAR), i.e. energy converting into heat in the medium per unit mass and time, increases, too. Considering mobile phones, this means that more energy from the phone will be absorbed in the user""s head.
An object of the invention is to alleviate the above-mentioned disadvantage associated with the prior art. A planar antenna according to the invention is characterized in that which is specified in the independent claim 1. A radio device according to the invention is characterized in that which is specified in the independent claim 14. Advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: a PIFA-type antenna is provided with a coaxial feed. This means that for the distance between the radiating plane and the ground plane the feed conductor of the radiating plane is surrounded by a shield conductor galvanically connected to the ground plane. The shield conductor at the same time functions as a short circuit conductor of the antenna. Antenna is matched by means of a matching slot between the connecting points of the feed and short circuit conductors and/or appropriate shaping of the short circuit conductor.
An advantage of the invention is that a feed arrangement according to it increases antenna gain without increasing the SAR value of the antenna. Thus, while the far field strength increases, the near field strength of the antenna, however, will not increase. If the trasmitting power of the antenna is decreased by an amount corresponding to the increase in gain, there is achieved a far field level equal to that of the prior art, but with a lower SAR value. Another advantage of the invention is that a structure according to it is relatively simple and inexpensive to fabricate.